1. Field of the Invention
The present invention relates to the structure of a pixel region of an active matrix display device and, more specifically, to the structure of a black matrix (BM) and auxiliary capacitors that are connected in parallel to respective pixel electrodes.
Further, the invention generally relates to the structure of a pixel region of flat panel displays that require a black matrix.
2. Description of the Related Art
Active matrix liquid crystal display devices are known, in which at least one thin-film transistor is connected to each of a number of pixel electrodes which are arranged in a matrix form, and charging, and discharging of each pixel electrode is controlled by the associated thin-film transistor.
Each pixel electrode constitutes a capacitor together with a counter electrode that is opposed to the pixel electrode with a liquid crystal therebetween.
In actual operation, however, since the capacitance of the capacitor constituted by each pixel electrode portion is not sufficiently large, an auxiliary capacitor is desired. However, when an electrode of the auxiliary capacitor is made of a conductive material such as a metal, it acts as a light-shielding portion in each pixel, resulting in a decrease of the aperture ratio.
On the other hand, a light-shielding member called “black matrix” is needed around each pixel electrode.
In general, regions where source lines (wiring lines for supplying currents to the respective source regions of the thin-film transistors) and gate lines (wiring lines for applying signal voltages to the respective gate electrodes of the thin-film transistors) are arranged in a matrix form have an uneven surface. As a result, a rubbing operation on an orientation film can not be performed properly in those regions, so that the orientation of liquid crystal molecules may be disordered there. This may cause a phenomenon that light leaks undesirable or, conversely, a desired quantity of light is not transmitted around the pixels. Further, the liquid crystal may be disabled from performing a desired electro-optical operation in the above regions.
If the above phenomenon occurs, a displayed image is blurred around the pixels, to thereby impair clearness of the image as a whole.
There is known a structure for solving the above problems in which structure a light-shielding film is so formed as to cover the edges of the pixel electrodes. The light-shielding film is called “black matrix” (BM).
U.S. Pat. No. 5,339,151 discloses a structure including a black matrix. In this technique, as shown in FIG. 2C of this patent, a black matrix extending from gate lines is so formed as to overlap with the edges of pixel electrodes. This structure is adapted to form auxiliary capacitors in overlapping portions of the black matrix and the pixel electrodes.
However, the technique of this patent has the following two problems. First, since the black matrix extends from the gate lines. First shielding is not complete. This is because source lines cannot be overlapped with the black matrix to avoid crosstalk. Light leakage should be tolerated in those portions.
Second, since the black matrix occupies the same plane as the gate lines, naturally it cannot perform light shielding for the sate lines themselves. Further, the black matrix cannot perform light shielding for the source lines to avoid crosstalk as described above.
With the recent development of digital equipments, influences of electromagnetic waves from low-frequency waves to microwaves have come to cause problems. In an environment where a liquid crystal electro-optical device is used, there is concern about the influences of electromagnetic waves.
Therefore, liquid crystal display devices need to be so constructed as to be insensitive to external electromagnetic waves.
In view of the above, the above structure as described in U.S. Pat. No. 5,339,181 in which the source lines and sate lines which transmit image signals are exposed to external electromagnetic waves is not preferable, because the source lines and gate lines may serve as an antenna.